In order to solve exhaustion of fossil energy and environmental problems due to using fossil energy, research into alternative energy source such as solar energy, wind energy, and hydro energy that is renewable and clean has been actively conducted.
Of those, an interest in a solar cell directly converting solar light into electric energy has significantly increased. Here, the solar cell means a cell generating current-voltage using a photovoltaic effect in which the cell absorbs light energy from solar light to generate electrons and holes.
Recently, a n-p diode type single-crystalline silicon (Si) based solar cell having photoelectric conversion efficiency higher than 20% may be manufactured and actually has been used in solar power generation, and a solar cell using a compound semiconductor such as GaAs having conversion efficiency higher than that of the n-p diode type single-crystalline silicon (Si) based solar cell is present. However, since these inorganic semiconductors based solar cells require a significantly high-purity purified material for high efficiency, a large amount of energy is consumed in purifying a raw material, and expensive processing equipment is required during a single crystallization process or a thinning process using the raw material, such that there is a limitation in lowering a manufacturing cost of the solar cell, thereby blocking large-scale use of the solar cell.
Therefore, in order to manufacture the solar cell at low cost, a cost of a core material used in the solar cell or the manufacturing process of the solar cell should be greatly reduced, and research into a dye-sensitized solar cell (DSSC) and an organic solar cell that may be manufactured using an inexpensive material and process has been actively conducted as an alternative to the inorganic semiconductor based solar cell.
The dye-sensitized solar cell (DSSC) was initially developed by Michael Gratzel in 1991, a professor at EPFL in Switzerland and was reported in Nature (Vol 353, P. 737).
An early DSSC had a simple structure in which a dye absorbing light was absorbed on porous photo-anodes on a transparent electrode film through which light and electricity flow, another conductive glass substrate was positioned under the film, and a liquid electrolyte was filled therebetween.
An operation principle of the DSSC is as follows. When dye molecules chemically absorbed on surfaces of the porous photo-anodes absorb solar light, the dye molecules generate electron-hole pairs, and electrons are injected into a conduction band of semiconducting oxides used as the porous photo-anodes to be transported to the transparent conductive film, thereby generating current. The holes remaining in the dye molecules configure of complete solar cell circuits in a shape in which the holes are transported to photo-cathodes by hole conduction caused by oxidation-reduction reaction of a liquid or solid electrolyte or hole-conductive polymer, thereby performing external work.
In this DSSC configuration, the transparent conductive film was mainly made of fluorine doped tin oxide (FTO) or indium doped tin oxide (ITO), and nanoparticles having a broad band gap are used as the porous photo-anodes. In this case, a factor to be firstly considered at the time of selecting nano semiconducting oxides (photo-anodes) for the DSSC is an energy value of the conduction band. Oxides studied up to now is mainly TiO2, SnO2, ZnO, Nb2O5, or the like. It is known that a material having the highest efficiency among these materials up to now is TiO2.
As the dye, various materials capable of absorbing light particularly well and easily separating an exciton generated by the light since a lowest unoccupied molecular orbital (LUMO) energy level of the dye is higher than an energy level of the conduction band of the photo-anode material to thereby increase the efficiency of the solar cell are chemically synthesized and used. The maximum efficiency of a liquid type DSSC reported up to now has been 11 to 12% for 20 years. The liquid type DSSC has relatively high efficiency to thereby make it possible to be commercialized. However, there are problems in stability according to time by a volatile liquid electrolyte and reducing cost due to using a high-cost ruthenium (Ru) based dye.
In order to solve these problems, research into uses of a non-volatile electrolyte using ionic solvent rather than the volatile liquid electrolyte, a gel-type polymer electrolyte, and an inexpensive pure organic dye has been conducted, but efficiency of a DSSC using these materials is lower than that of the DSSC using the volatile liquid electrolyte and Ru based dye.
Meanwhile, the organic photovoltaic (OPV) that has been studied in earnest since the mid-1990 is configured of organic materials having electron donor (D, or often called a hole acceptor) characteristics and electron acceptor (A) characteristics. When the solar cell made of organic molecules absorbs the light, electrons and holes are formed, which are called exciton.
The exciton moves to a D-A interface, such that an electric charge is separated, an electron moves to the electric acceptor, and the hole moves to the electron donor, thereby generating photo current. Examples of combination of materials mainly used in the organic photovoltaic include an organic (D)—fullerene (A) based material, an organic (D)—organic (A) based material, an organic (D)—nano-inorganic (A) based material, and the like.
Since a distance at which the exciton generated in the electron donor may move is about 10 nm, which is significantly short, photo active organic materials may not be thickly laminated, such that optical absorption spectra was low and the efficiency was low. However, recently, due to introduction of so-called bulk heterojunction (BHJ) concept of increasing a surface area at an interface and development of an electron donor organic material having a small band gap to easily absorb solar light of a wide range, the efficiency was significantly increased, such that an organic photovoltaic having efficiency of 6.77% has been reported (Nature Photonics, Vol3, p. 649).
In the organic photovoltaic, a manufacturing process of a cell is simple due to high formability of the organic material, diversity thereof, and a low cost thereof, such that the organic photovoltaic may be manufactured at a low cost, as compared to the existing solar cell. However, the organic photovoltaic has a problem that a structure of BHJ is degraded by moisture in air or oxygen to rapidly decrease the efficiency thereof, that is, a problem in the stability of the solar cell. When a technology of completely sealing the solar cell is introduced in order to solve this problem, the stability may be increased, but a cost may also be increased.
As a method of solving a problem of the DSSC by the liquid electrolyte, an all-solid state DSSC using Spiro-OMeTAD[2,22′,7,77′-tetrkis (N,N-di-p-methoxyphenylamine)-9,99′-spirobi fluorine], which is a solid state hole conductive organic material rather than the liquid electrolyte to have efficiency of 0.74% was reported in Nature (Vol 395, p. 583) in 1998 by Michael Gratzel, a chemistry professor at EPFL in Switzerland, who is an inventor of the DSSC. Afterward, the efficiency was increased up to about 5.0% by optimizing the structure, improving interfacial properties, and improving hole conductivity. In addition, a solar cell using the inexpensive pure organic dye instead of the ruthenium based dye and using P3HT, PEDOT, or the like as a hole conductor has been manufactured, but efficiency of the solar cell is still low, at 2 to 4%. Recently, a fact that a cell using SQ1{5-carboxy-2-[[3-[(1,3-dihydro-3,3-dimethyl-1-ethyl-2H-indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-trimethyl-1-octyl-3H-indolium} dye absorbed in a nanotubular TiO2 and using P3HT as the hole conductor may have maximum efficiency of 3.2% has been reported [Nano Letters, 9, (2009) 4250]. However, there was a problem in the stability of the cell such as a problem that efficiency of the cell was reduced by half after 3 days, or the like.
Further, research into a solar cell using quantum dot nanoparticles as a light absorber rather than the dye and using hole conductive inorganic material or organic material rather than the liquid electrolyte has been reported. A fact that a cell using CdSe (CdTe surface coating) as the quantum dot and using spiro-OMeTAD as the hole conductive organic material has efficiency of about 1.8% at weak light ( 1/10 of the solar light intensity) has been reported [Nano letters, 9, (2009) 4221]. In this case, the cell has excessively low efficiency in addition to a problem generated by using CdSe which contains toxic Cd.
In addition, a fact that a solar cell using Sb2S3 as a light absorbing inorganic material and using CuSCN as the hole conductive inorganic material has efficiency of 3.37% has been reported [J. Phys. Chem. C, 113 (2009) 4254]. However, in this case, the CuSCN, which is an inorganic hole conductor, and Sb2S3, which is a light absorber, are reacted with each other to generate CuS, which causes efficiency to be rapidly decreased as time goes by.
The reasons why the quantum dot nano particle is used as the light absorber in the solar cell field are as follows. 1) A thickness of a photo electrode required to completely absorb the solar light in a sensitized solar cell may be reduced due to a high light absorption coefficient. 2) Since light absorption band gap may be easily controlled by controlling composition or a particle size of the quantum dot nano particle, the quantum dot nano particle may be utilized as a light-sensitive material absorbing up to near infrared light. 3) Multilayer coating of the quantum dot nano particle and hybridization thereof with the dye may be performed. 4) The photo current may be increased by multiple exciton generation, thereby making it possible to remarkably improve the efficiency. In addition, since the quantum dot nano particle is an inorganic material, the stability thereof for the light may be more excellent as compared to the dye made of an organic material.
However, each of the organic solar cell based on an organic semiconductor, the DSSC based on an organic/inorganic dye, and the inorganic solar cell based on an inorganic semiconductor has been individually researched up to now, but research into and development of “an all-solid state nanostructured inorganic-organic heterojunction solar cell” expected to have all of the high efficiency and the stability and be manufactured at low cost by combining an advantage of the inorganic thin-film solar cell capable of easily absorbing solar energy with a wide band from visible light to near infrared light and an advantage of the organic solar cell capable of being manufactured at low cost by a solution process with a structure of the DSSC that may be cheap and have high efficiency has not been conducted at all. Further, when a nano-encapsulated quantum dot of the inorganic semiconductor used in the present invention, an advantage of the quantum dot may be combined.